Mach number, dynamic pressure and airspeed are typically used to set the gain of autopilots and measuring these parameters typically relies on devices that must extend into the free airstream. For example, pitot tubes determine dynamic pressure by measuring the difference between free airstream total pressure and static pressure. The extension of these tubes into the free airstream can be an inconvenience, particularly in rocket and munition applications. In addition, the tube can become clogged with debris impacting the accuracy of the measurements from which one can calculate gains for an autopilot.
Autopilots have been used in the control and guidance of ordinances including rockets, mortars and the like. The purpose of the autopilot is to maintain stable flight and in order to do so various gain parameters of the autopilot must be set. The gain parameters are critically dependent upon dynamic pressure and heretofore preprogrammed flight of these munitions has been used to predict the dynamic pressure that will be encountered by the munition which varies with time over the trajectory of the munition. Thus, in prior systems guidance for these munitions used preprogrammed scheduling involving gain parameters that varied over time in accordance with a flight profile as well as being specific to a particular munition. The scheduling is heavily dependent on prediction of flight path and flight characteristics and does not take into account that differing flight paths are possible. It will therefore be appreciated that real time measurement of dynamic pressure would significantly improve the system performance since one would not have to rely on predicted gains from scheduling.
When these munitions are helicopter platform launched, their speeds are relatively low and any prediction of gain over time in this type of very constrained engagement scenario is relatively easy. However, if these autopilots are employed on munition fired from a fixed wing aircraft or jet aircraft flying at Mach speed, then a very dynamic condition may be obtained for which a time schedule is hard to predict, especially one that fits a range of platforms including both helicopters and jet aircraft.
Using a time based gain schedule assumes that all munitions fly the same profile and encounter the same sequence of events so that the schedule of the gains would be all exactly the same. However, the fact is that as time goes on, deviation further and further from the trajectory assumption occurs, and performance starts to suffer.
Moreover, for certain types of munitions the rocket or projectile is launched almost vertically and the time variation of the sequence of dynamic pressure and gain is a complicated process to predict. This is particularly true for a mortar type munition that follows a parabolic path and can be launched over a large range of initial velocities and elevation angles.
Dynamic pressure is a key measurement to the setting of gains for the autopilot. In the past, one method of measurement of dynamic pressure took place from the exterior of the munition using pitot tubes. However these pitot tubes and other sensors that project beyond the skin of the munition or vehicle oftentimes produce unreliable results. Moreover, sensors mounted to the exterior of a munition can be damaged in transport or during launch under a high G environment. Moreover, when a rocket is launched out of a tube, with a neighboring rocket launched just before, the adjacent rocket is subjected to a backwash of exhaust fumes that can potentially clog the pitot tube and therefore change its performance characteristic.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.